1. Field of the Invention
This invention relates to a device for handling wafers with every group of more than one that may be built in a cassette-less automatic wafer cleaning apparatus which does not require very high accuracy in positioning the wafers and may accommodate the wafers of different diameters.
2. Description of the Prior Art
In semiconductor device fabrication, semiconductor devices as final product is adversely affected in performance thereof by impurities or foreign matter stuck on the surface of a wafer made of semiconductor single crystal (hereinafter referred to as wafer), which is a starting material for the devices.
A cleaning operation is therefore indispensable in processes for producing wafers. And there are available a variety of cleaning methods in the cleaning operation for a process for producing wafers, which are most broadly classified into the two group of physical methods and chemical methods.
In the physical methods, included are a method in which impurities stuck on the surface of a wafer are directly removed by the mechanical action of a washing brush, another method in which a pressurized liquid is shot in a stream onto part or all of the surface of a wafer from a jet nozzle and thereby the impurities are removed off, and still another method in which impurities stuck on the surface of a wafer are removed off by application of ultrasonic waves (which is the so-called ultrasonic cleaning method).
In the chemical methods, included are methods in which impurities stuck on the surface of a wafer are chemically decomposed into constituents to be removed by the powers of chemicals or enzymes. In the meantime, both of the methods may be combined so as to improve the total cleaning capacity.
A trend of increasingly more complexity of a semiconductor device in recent years demands wafers as starting material of an increasingly larger diameter in semiconductor device fabrication, so that laborious work should be required in a cleaning operation if a conventional method was adopted in which wafers are contained in a plastic cassette as carrier and washed as they are kept therein.
What's more, another problem is brought about that undesirable particles would be stuck onto wafers to pollute due to collision or friction between the wafers and the cassette, which produces the particles in a transportation system where the wafers are contained within the cassette.
The present applicant has proposed a single-wafer type cassette-less automatic wafer cleaning apparatus, which is disclosed in First Publication HEI No. 5-36667, wherein wafers are dealt with one at a time to be entirely immersed into cleaning solutions respectively held in a plurality of cleaning baths for washing. The applicant has also proposed a wafer handling device that is designed to handle wafers with one at a time, which is disclosed in First Publication HEI No. 5-47729.
The above-mentioned single-wafer type cassette-less automatic wafer cleaning apparatus has been well received by those working in the field of the art, since laborious work needed is greatly reduced in comparison with the same needed by a conventional method, wherein a plurality of wafers are contained in a carrier cassette for washing, and on the top of the above merit the conventional wafer pollution problem caused by a carrier cassette itself during transportation is completely settled.
However, the above-mentioned single-wafer type cassette-less automatic wafer cleaning apparatus still have a problem that increase in process efficiency could not be expected very much due to the way of handling wafers one at a time therein.
The inventors have therefore studied in pursuit of increase in process efficiency of the cassette-less automatic wafer cleaning apparatus and as a result reached to the present invention. In the course of the study, a notion has been being kept in mind that a possibility that the process efficiency could be increased by handling wafers with every group of more than one instead of one at a time.